The following
interesting explanation of a
DiffieHellman key exchange, including a 2line (
!)
Perl script to implement it, is from:
http://www.cypherspace.org/~adam/rsa/perldh.html
It is by Adam Back. (
Munged email: <not eve but her partner ____> at cypherspace dot org).
Please
note that I had to
fudge a lot of things in it (some <pre>
tags wanted to treat < as opening an
HTML tag, and some didn't.) So if there's anything
weird, lemme' know. (Unless I've
just noded this...I have
a sneaking suspicion that the
Scratch Pad isn't
quite WYSIWYG...in which case, I am already busily working to fix it!).
Beginquote
DiffieHellman in 2 lines of perl
#!/usr/local/bin/perl  exportacryptosystemsig DiffieHellman2lines
($g,$e,$m)=@ARGV,$mdie"$0 gen exp mod\n";print`echo "16dio1[d2%Sa2/d0<X+d
*La1=z\U$m%0]SX$e"[$g*]\EszlXx+pdc`
DiffieHellman key exchange allows two parties who have not met to
exchange keys securely on an unsecure communication path. Typically
DH is used to exchange a randomly generated conventional encryption
key, the rest of the exchange is then encrypted with the conventional
cipher. It has been used with DES, 3DES, IDEA, RC4 though basically
the approach of using DH key exchange can be used for any
conventional stream or block cipher.
PGP itself operates in a similar fashion, except that PGP uses RSA for
key exchange, and IDEA as the conventional cipher.
The maths for DiffieHellman is quite simple.
Here is an example:
We are trying to exchange a random key for communication. Say that we
will be using the RC4 stream cipher as our conventional cipher.
Here's the stages in the process.

You advertise your public generator g, and public prime modulus m as
your "public key" in the same way that you would advertise an RSA
public key, you give this to anyone you wish to exchange messages
with. This is no requirement for these numbers to be kept secret, all
knowledge of these numbers gives is the ability to send you messages.
You do not tell anyone what your secret exponent x is.
You initiate a key exchange by calculating a:
x
a = g mod m
and sending me the number a.
You can use the dh perl program to do this calculation. For the
sake of argument we will choose some numbers. Say that your public
generator g = 3, your public prime modulus m = 10001 and your secret
exponent x = 9a2e (all numbers in hex). Then you would run the perl
dh program like this to calculate a:
% dh g x m
You send me the following in email:
g = 3, m = 10001, a = c366

Now I choose a session key to use for RC4, using DH key exchange
I can send this to you without an eavesdropper being able to get the
key.
I do not choose a session key directly but rather choose a random
number y from which I will calculate the session key.
Say that the random y = 4c20.
Now I calculate the session key s:
y
s = a mod m
Or using the perl dh program:
% dh c366 4c20 10001
So the session key s = ded4

In order to send you the session s key I need only calculate:
y
b = g mod m
Again using the perl dh program:
% dh 3 4c20 10001
So b = 6246
I send the number b to you in email, now you can recover the session
key:
x
s = b mod m
So to obtain the session key, using the perl dh program you do:
% dh 6246 9a2e 10001
And sure enough out pops the session key as I calculated, s = ded4.
Now we both have the session key and commence exchanging messages
using our agreed key as the key to a symmetric cipher such as rc4.
So now I can send you (or you can send me) rc4 messages encrypted with
our negotiated session key s = ded4:
% cat msg  rc4 ded4  uuencode r r  mail
A perl (and a C version) of rc4 suitable for the above can be found
here.
This works because of the mathematical property that:
y x
x y
g mod m = g mod m
and
x y
a = g mod m, b = g mod m
y x
=> s = a mod m = b mod m
x and y never change hands, and yet s has changed hands. Further you
can't discover my y and I can't discover your x, and an eavesdropper
has neither x nor y and so cannot discover s.
Real example
You can try sending me some RC4 encrypted email, using a DH
negotiated session key.
For security (the above example is for clarity only 32 bit keys are
utterly useless for security purposes) we will use 1024 bits. Here is
my DiffieHelman public key, you can have a go at negotiating a DH key
exchange and sending me some RC4 encrypted email.
I have chosen an x (which I won't be telling you this time for obvious
reasons), and calculated the corresponding a as described above. Here
are my DH public key numbers:
g = 3
m =
de9b707d4c5a4633c0290c95ff30a605aeb7ae864ff48370f13cf01d49adb9f2
3d19a439f753ee7703cf342d87f431105c843c78ca4df639931f3458fae8a94d
1687e99a76ed99d0ba87189f42fd31ad8262c54a8cf5914ae6c28c540d714a5f
6087a171fb74f4814c6f968d72386ef356a05180c3bec7ddd5ef6fe76b0531c3
a =
56C03667F3B50335AD532D0ADCAA2897A02C0878099D8E3AAB9D80B2B5C83E2F
14C78CEE664BCE7D209E0FD8B73F7F6822FCDF6FFADE5AF2DDBB38FF3D2270CE
BBED172D7C399F47EE9F1067F1B85CCBEC8F43B721B4F9802F3EA51A8ACD1F6F
B526ECF4A56AD62B0AC17551727B6A7C7AADB9362394B410611A21A7711DCDE2
To send me some mail you will need to generate your choice of random y
as described above, and then calculate the session key s:
% dh [a] [y] [m] > s
(where [a] and [m] are the large numbers above cut and pasted in, and
[y] is your large random number.)
and then calculate key exchange number b:
% dh [g] [y] [m] > b
% mail adam@cypherspace.org < b
Then you compose your mail message as file "msg" and rc4 encrypt that
using s as calculated above:
% cat msg  rc4 `cat s`  uuencode r r  mail adam@cypherspace.org
Comments, html bugs to me [contact info. as at top of node.]
Endquote